The present invention relates generally to methods of forming filament reinforced matrix metal composites. More particularly, the present invention relates to a method by which a silicon carbide filament reinforcement may be incorporated effectively and efficiently in a composite having a titanium based metal matrix.
It is known that filament reinforced composite structures having matrices of titanium based metals can be formed employing plasma spray deposition techniques. A number of U.S. Patents relating to this general subject and assigned to the assignee of the subject application have been issued. The preparation of titanium alloy base foils, sheets, and similar articles and of reinforced structures in which silicon carbide fibers are embedded in a titanium base alloy are described in U.S. Pat. Nos. 4,775,547; 4,782,884; 4,786,566; 4,805,294; 4,805,833; and 4,838,337; assigned to the same assignee as the subject application. The texts of these patents are incorporated herein by reference.
Preparation of composites as described in these patents is the subject of intense study inasmuch as the composites have very high strength properties in relation to their weight. One of the properties which is particularly desirable is the high tensile properties imparted to the structures by the high tensile properties of the silicon carbide fibers or filaments. The tensile properties of the structures are related to the rule of mixtures. According to this rule the proportion of the property, such as tensile property, which is attributed to the filament, as contrasted with the matrix, is determined by the volume percent of the filament present in the structure and by the tensile strength of the filament itself. Similarly, the proportion of the same tensile property which is attributed to the matrix is determined by the volume percent of the matrix present in the structure and the tensile strength of the matrix itself.
Prior to the development of the processes described in the above-referenced patents, such structures were prepared by sandwiching the reinforcing filaments between foils of titanium base alloy and pressing the stacks of alternate layers of alloy and reinforcing filament until a composite structure was formed. However, that prior art practice was found to be less than satisfactory when attempts were made to form tube or ring structures in which the filament was an internal reinforcement for the entire tube or ring.
The structures taught in the above-referenced patents and the methods by which they are formed, greatly improved over the earlier practice of forming sandwiches of matrix and reinforcing filament by compression.
Later it was found that while the structures prepared as described in the above-referenced patents have properties which are a great improvement over earlier structures, the attainment of the potentially very high ultimate tensile strength of these structures did not measure up to the values theoretically possible. The testing of composites formed according to the methods taught in the above patents has demonstrated that although modulus values are generally in good agreement with the rule of mixtures predictions, the ultimate tensile strength is usually much lower than predicted by the underlying properties of the individual ingredients to the composite. A number of applications have been filed which are directed toward overcoming the problem of lower than expected tensile properties and a number of these applications are copending. These include applications Ser. No. 445,203, filed Dec. 4, 1989; Ser. No. 459,894, filed Jan. 2, 1990; and Ser. Nos. 455,041 & 455,048, both filed Dec. 22, 1989. The texts of these applications are incorporated herein by reference.
One of the structures which has been found to be particularly desirable in the use of the technology of these reference patents is an annular article having a metal matrix and having silicon carbide filament reinforcement extending many times around the entire annulus. Such tubular or ring structures have very high tensile properties relative to their weight, particularly when compared to structures made entirely of metal. Such structures must be precise in their internal dimensions in order for the structures to be used most effectively in end use applications inasmuch as the structures are often used as part of a more complex structure and for this purpose are fitted over one or a number of elements in a circular form in order to serve as a reinforcing ring.
One of the structures which is formed has the reinforcing filament wound many times and in many layers around the circumference is a reinforced ring or tubular structure. A reinforced ring can be used for example as a reinforcment, for an integrally bladed compressor disk of a jet engine. In order to serve to hold the blades in a compressor stage of a jet engine a large number of layers of reinforcing filaments are required.
It has been found that it is very difficult to continue to add more and more layers of filament reinforcement to a tubular or ring structure because of differences in thermal expansion coefficient and other factors. One way in which this problem has been solved is explained in copending application Ser. No. 546,228 filed Jun. 29, 1990 now U.S. Pat. No. 4,981,643 dated Jan. 1, 1991. The method disclosed in the copending application involves forming a series of concentric rings which are then assembled together to provide a reinforced ring structure having more than 100 layers of reinforcement. Such ring structures are of quite large diameter on the order of a foot or several feet and must nevertheless be nested together within very close tolerances.
While the prior art teachings of the issued patents relates generally to the successful formation of reinforced composite structures having silicon carbide filaments embedded in matrix metal environments, the processes by which such structures were formed were not totally reproducible and reliable to a degree which is entirely suitable for a manufacturing process. Part of the difficulty associated with the formation of composite structures by the teaching of the prior patents related to the spacing of the filaments. It was found, for example, that where the deposit of matrix left an uneven surface, the spacing of the plasma spray filamentary reinforcement tended also to be aligned in an uneven pattern so that in some places the filaments touched and in other places the filaments were too widely separated. Where the filaments touched in groups of two, three or four or more strands, the inadequate spacing between the strands of filaments resulted in an inadequate penetration of the filaments by the plasma spray matrix metal so that the metal did not pass between the filaments and form a bond with the metal therebeneath. Rather the plasma sprayed metal would deposit on top of the group of abutting filament strands and would leave a void there beneath which void had to be later filled to the degree possible by a consolidation process. However, such uneven spacing resulted in too dense a spacing of reinforcing filament in some locations and too sparse a spacing of filaments in other locations of the composite product.